The invention relates to a protease for activating the blood clotting factor VII, to a process for isolating it, detecting it and inactivating it, and to medicinal preparations which comprise this protease.
The blood dotting system comprises two different, cascade-like pathways for activating clotting factors which are present in the plasma. The intrinsic or the extrinsic pathway is preferentially used for initiating clotting, depending on the triggering mechanism.
When a tissue is damaged, thromboplastin (tissue factor, TF with phospholipids) is exposed by the affected cells as the starter of the extrinsic cloning pathway. The membrane-located thromboplastin can bind both cloning factor VII (FVII) and circulating, activated FVII (FVIIa). In the presence of calcium ions and lipids, this TF-FVIIa complex leads to the binding of FX, which is converted into its activated form (FXa) by limited pioteolysis. FXa in turn leads, by activating prothrombin to form thrombin, to the formation of fibrin and thereby ultimately to closure of the wound.
While the further activation of the thromboplastin-bound FVII initially takes place autocatalytically, in particular, it is supported, after the clotting cascade has been initiated, by FXa and thrombin, in particular, leading to marked reinforcement of the reaction cascade.
The administration of FVIIa or FVIIa-containing concentrates is indicated in certain clinical situations. The so-called FVIII-bypassing activity (FEIBA) of FVIIa is used in patients who are suffering, for example, from hemophilia A and have developed antibodies against FVIII as a consequence of the administration of FVIII. According to presently available findings, FVIIa is well tolerated in this context and, while it does not lead to any tendency to thrombosis, it is suitable for ensuring that clotting takes place to a limited but adequate extent. Recombinant FVIIa is already being used therapeutically and prophylactically. FVII which has been isolated from blood plasma can also be activated and then used. Proteases such as thrombin can be used for this activation; however, these proteases, as such, can themselves strongly activate clotting and lead to the risk of a thrombosis. For this reason, subsequent removal or inactivation of thrombin is necessary and leads to yield losses. As a result of the risk of thrombosis which is associated with it, the use of FXa or FIIa (thrombin) is frequently contraindicated and only indicated in emergencies, e.g. in association with extreme loss of blood and unstaunchable hemorrhages.
FVIIa is found in very low concentrations in the plasma of healthy subjects. Only very little is so far known about the formation and origin of FVIIa which is circulating in the blood. Traces of thromboplastin which has been expressed or released in association with cell destruction might play a role in this context. Although it is known that factor XIIa, for example, can lead to FVII activation under certain conditions, the physiological relevance of this reaction has not yet been clarified.